You can't poison, pollute and harvest the world's oceans forever without there being consequences. Granted, the oceans are vast, covering almost three-quarters of the Earth's surface, so consequences are slow in coming, but they are inevitable.

A new study by the World Wildlife Fund has reported that fully half of marine life has been killed off in the last four decades.

"In the space of a single generation, human activity has severely damaged the ocean by catching fish faster than they can reproduce," Marco Lambertini, head of WWF International, said in a statement. "Profound changes are needed to ensure abundant ocean life for future generations."

WWF called on world leaders to prioritize ocean recovery when the United Nations' 15-year Sustainable Development Goals are approved later this month.

The report said protected global ocean area -- currently about 3.4 percent -- should be tripled by 2020. It also called on consumers and fish retailers to source from companies that follow certified "best practice" standards.

"These changes are happening in our lifetime," Lambertini said. "We can and we must correct course now."